semctl - semaphore control operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semctl(int semid, int semnum, int cmd, union semun arg);
The semctl() system call provides a number of control operations on the
semaphore specified by semnum and semid. The operation to
be performed
is specified in cmd (see below). arg is a union of the following fields:
int val; /* value for SETVAL
*/
struct semid_ds *buf; /* buffer for
IPC_{STAT,SET} */
u_short *array; /* array for GETALL
& SETALL */
The semid_ds structure used in the IPC_SET and IPC_STAT
commands is defined
as follows in <sys/sem.h>:
struct semid_ds {
struct ipc_perm sem_perm; /* operation permissions */
struct sem *sem_base; /* semaphore set */
u_short sem_nsems; /* number of sems in
set */
time_t sem_otime; /* last operation
time */
time_t sem_ctime; /* last change time
*/
};
The ipc_perm structure used inside the semid_ds structure
is defined in
<sys/ipc.h> and looks like this:
struct ipc_perm {
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* r/w permission (see
chmod(2)) */
u_short seq; /* sequence # (to generate
unique msg/sem/shm id) */
key_t key; /* user specified
msg/sem/shm key */
};
semctl() provides the following operations:
GETVAL Return the value of the semaphore.
SETVAL Set the value of the semaphore to arg.val.
GETPID Return the pid of the last process that did an
operation on
this semaphore.
GETNCNT Return the number of processes waiting to acquire
the
semaphore.
GETZCNT Return the number of processes waiting for the
value of the
semaphore to reach 0.
GETALL Return the values for all the semaphores associated with
semid.
SETALL Set the values for all the semaphores that are
associated with
the semaphore identifier semid to the corresponding values in
arg.array.
IPC_STAT Gather statistics about a semaphore and place the
information
in the semid_ds structure pointed to by arg.buf
(see above).
IPC_SET Set the value of the sem_perm.uid, sem_perm.gid
and
sem_perm.mode fields in the structure associated
with the
semaphore. The values are taken from the corresponding fields
in the structure pointed to by arg.buf. This operation can
only be executed by the superuser, or a process
that has an
effective user ID equal to either sem_perm.cuid
or
sem_perm.uid in the data structure associated
with the message
queue.
IPC_RMID Remove the semaphores associated with semid from
the system
and destroy the data structures associated with
it. Only the
superuser or a process with an effective UID
equal to the
sem_perm.cuid or sem_perm.uid values in the data
structure associated
with the semaphore can do this.
The permission to read or change a message queue (see semop(2)) is determined
by the sem_perm.mode field in the same way as is done
with files
(see chmod(2)), but the effective UID can match either the
sem_perm.cuid
field or the sem_perm.uid field, and the effective GID can
match either
sem_perm.cgid or sem_perm.gid.
For the GETVAL, GETPID, GETNCNT, and GETZCNT operations,
semctl() returns
one of the values described above if successful. All other
operations
will make semctl() return 0 if no errors occur. Otherwise
-1 is returned
and errno set to reflect the error.
semctl() will fail if:
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the
caller is not
the superuser, nor does the effective UID
match either the
sem_perm.uid or sem_perm.cuid fields of the
data structure
associated with the message queue.
[EACCES] The caller has no operation permission for
this semaphore.
[EINVAL] semid is not a valid message semaphore identifier.
cmd is not a valid command.
[EFAULT] arg.buf specifies an invalid address.
semget(2), semop(2)
OpenBSD 3.6 August 17, 1995
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